In my QGIS 3.4.1 installation (Windows 10) the extensions repository says
"unavailable", like in this older post: Why I can't install plugins?.
I already added the repository with
"HTTP" instead of
"HTTPS", but it shows the same error.
I changed nothing on my network installation, so a proxy-problem should not be the reason.
In my older QGIS 2.18.16 installation, the repository works just fine.
Maybe anyone has a suggestion how to fix the problem?